Structural Reliability of Dapped End Beams with Different Reinforcement Layouts under Dynamic Loading

Reinforced concrete Dapped End Beams (DEB), also known as half-joints, are used in bridges and many other pre-cast constructions to reduce end depth and increase lateral stability. Dapped end beams are expected to experience dynamic loads when used in bridges, the available past studies on the behavior and damage assessment of DEBs are mainly for static loading. The reinforcement layouts of DEBs can influence the behavior of these shear critical members under impact loading. Better understanding of the crack propagation and failure patter of DEBs under dynamics loads is required for safe and economic design of these structural elements. A non-linear numerical transient study was conducted to investigate the dynamic performance of DEBs with different reinforcement layouts. Advanced material models capable of including strain-rate effect and material non-linearity to capture realistic behavior of DEBs were used. The simulated models in finite element package LS-DYNA were verified and used to conduct detailed parametric study to investigate the impact behavior of DEBs with different reinforcement layouts. Sensitivity of concrete compressive strength, main dapped end reinforcement and special shear reinforcement detailing on the structural reliability of DEBs were studied.</p

3D meso-scale modelling of concrete material in spall tests

Tensile strength is one of the key factors of concrete material that need be accurately defined in analysis of concrete structures subjected to high-speed impact loads. Dynamic tensile strength of concrete material is usually obtained by conducting laboratory tests such as direct tensile test, Brazilian splitting test and spall test. Concrete is a heterogeneous material with different components, but is conventionally assumed to be homogeneous, i.e., cement mortar only, in most previous experimental or numerical studies. The aggregates in concrete material are usually neglected owing to testing limitation and numerical simplification. It has been well acknowledged that neglecting coarse aggregates might not necessarily give accurate concrete dynamic material properties. In the present study, a 3D meso-scale model of concrete specimen with consideration of cement mortar and aggregates is developed to simulate spall tests and investigate the behaviour of concrete material under high strain rate. The commercial software LS-DYNA is used to perform the numerical simulations of spall tests. The mesh size sensitivity is examined by conducting mesh convergence tests. The reliability of the numerical model in simulating the spall tests is verified by comparing the numerical results with the experimental data from the literature. The influence of coarse aggregates on the experimental test results is studied. The wave attenuation in concrete specimen is analysed, and empirical equations are proposed for quick assessment of the test data to determine the true dynamic tensile strength of concrete material. The contributions of aggregates to dynamic strength in spall tests are quantified for modifying the test results based on mortar material in the literature

Influence of corners in excavations on damage assessment

This paper provides guidance on quantifying the extent of corner effects in excavations and their impact on damage assessment. The corner effects’ extent is of great importance in making early decisions during project planning and preliminary design, particularly in relation to stakeholder engagement and placement of instruments. By using empirical relations, one is able to provide an equation, validated against the literature and additional numerical models, for estimating the extent of corner effects for a particular excavation geometry. Furthermore, two more equations for quantifying the damage of excavations to adjacent structures are presented and validated against two case studies in the literature. The proposed equations are also useful in the context of early stages of project development. Finally, a simple study shows the different effects of corners in sections parallel and perpendicular to a retaining wall. This highlights that corner effects may actually induce additional damage due to the introduction of a movement gradient, as opposed to the common previous perception that assumed that they were always conservative as they reduced absolute movements

Earthquake response of a multiblock nuclear reactor graphite core:Experimental model vs simulations

The complex dynamics of a quarter-scale model of a graphite nuclear reactor core, representative of the second generation of British advanced gas-cooled nuclear reactors, is investigated numerically and experimentally. Advanced gas-cooled nuclear reactor cores are polygonal, multilayer, arrays of graphite bricks, with each brick allowed to rock by design relative to each other in accordance with the boundary conditions. A 35 000 DOF, nonlinear finite element model of the core created by Atkins Nuclear, was analysed on a high performance computing facility at the University of Bristol, and a corresponding 8 t physical model, equipped with 3200 data acquisition channels, was built and tested on the University of Bristol 6-DOF shaking table. In this paper, the two models are subjected to a series of (1) synthetic earthquake and (2) idealised harmonic input motions. The experimental data are used to compare and verify the two models and explore the dynamics of the core. A kinematic model of the response is also developed based solely on geometric constraints. The results are presented in the form of response maps and graphs. Important conclusions are drawn as to the dynamics and earthquake response of such systems, which inform numerical model validation. It is found that contrary to the case of a small number of rocking blocks that exhibit highly complex response patterns, the behaviour of the model at hand is both smooth and repeatable. An analogy between the response of the core and that of dense granular matter exhibiting particle interlocking and dilatancy is highlighted.</p

An experimental and numerical investigation of the effects of geometry and spot welds on the crashworthiness of vehicle thin-walled structures

This paper aims to develop a new crash box with improved crashworthiness at reduced cost and weight as a base design for use in the automotive industry. Firstly, a baseline crash box model as presently used by the automotive industry was comprehensively examined by numerical crash analysis using Ls-Dyna software.. Considering the initial design geometry, forty-five different crash box designs were developed by making changes in the geometry and wall thickness of the thin walled structures. The effects of the changes in wall thickness and geometry in alternative crash box designs on crash performances such as total energy absorption, peak crush force, mean crush force, specific energy absorption and crush force efficiency were investigated. The optimum crash box design obtained numerically was validated experimentally by means of the drop tower impact system. The numerical crash analysis results clearly agree with the experimental test results. In this study, a new crash box design at a lower cost and performing better in crashes compared with the other forty-six designs has been obtained and can be used in the automotive industry as an energy absorber. The results have revealed that crash box geometry, as well as the number and position of the spot welds and sheet-metal thickness have an important effect on crash performance, weight and cost of the crash boxes.Turkish Ministry of Science, Industry and TechnologyMinistry of Science, Industry & Technology - Turkey; TOKSAN RD Center [01348.STZ.2012-1]This study was funded by the Turkish Ministry of Science, Industry and Technology and TOKSAN R&D Center within the scope of the SAN-TEZ project 01348.STZ.2012-1